semop
(3p)

This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

The semop() function shall perform atomically a user-defined array of semaphore operations in array order on the set of semaphores associated with the semaphore identifier specified by the argument semid.

The argument sops is a pointer to a user-defined array of semaphore operation structures. The implementation shall not modify elements of this array unless the application uses implementation-defined extensions.

The argument nsops is the number of such structures in the array.

Each structure, sembuf, includes the following members:

Member Type

Member Name

Description

unsigned short

sem_num

Semaphore number.

short

sem_op

Semaphore operation.

short

sem_flg

Operation flags.

Each semaphore operation specified by sem_op is performed on the corresponding semaphore specified by semid and sem_num.

The variable sem_op specifies one of three semaphore operations:

1.

If sem_op is a negative integer and the calling process has alter permission, one of the following shall occur:

*

If semval(see <sys/sem.h>) is greater than or equal to the absolute value of sem_op, the absolute value of sem_op is subtracted from semval. Also, if ( sem_flg &SEM_UNDO) is non-zero, the absolute value of sem_op shall be added to the semadj value of the calling process for the specified semaphore.

*

If semval is less than the absolute value of sem_op and ( sem_flg &IPC_NOWAIT) is non-zero, semop() shall return immediately.

*

If semval is less than the absolute value of sem_op and ( sem_flg &IPC_NOWAIT) is 0, semop() shall increment the semncnt associated with the specified semaphore and suspend execution of the calling thread until one of the following conditions occurs:

--

The value of semval becomes greater than or equal to the absolute value of sem_op. When this occurs, the value of semncnt associated with the specified semaphore shall be decremented, the absolute value of sem_op shall be subtracted from semval and, if ( sem_flg &SEM_UNDO) is non-zero, the absolute value of sem_op shall be added to the semadj value of the calling process for the specified semaphore.

--

The semid for which the calling thread is awaiting action is removed from the system. When this occurs, errno shall be set to [EIDRM] and −1 shall be returned.

--

The calling thread receives a signal that is to be caught. When this occurs, the value of semncnt associated with the specified semaphore shall be decremented, and the calling thread shall resume execution in the manner prescribed in sigaction().

2.

If sem_op is a positive integer and the calling process has alter permission, the value of sem_op shall be added to semval and, if ( sem_flg &SEM_UNDO) is non-zero, the value of sem_op shall be subtracted from the semadj value of the calling process for the specified semaphore.

3.

If sem_op is 0 and the calling process has read permission, one of the following shall occur:

If semval is non-zero and ( sem_flg &IPC_NOWAIT) is 0, semop() shall increment the semzcnt associated with the specified semaphore and suspend execution of the calling thread until one of the following occurs:

--

The value of semval becomes 0, at which time the value of semzcnt associated with the specified semaphore shall be decremented.

--

The semid for which the calling thread is awaiting action is removed from the system. When this occurs, errno shall be set to [EIDRM] and −1 shall be returned.

--

The calling thread receives a signal that is to be caught. When this occurs, the value of semzcnt associated with the specified semaphore shall be decremented, and the calling thread shall resume execution in the manner prescribed in sigaction().

Upon successful completion, the value of sempid for each semaphore specified in the array pointed to by sops shall be set to the process ID of the calling process. Also, the sem_otime timestamp shall be set to the current time, as described in Section 2.7.1, IPC General Description.

The following example gets a unique semaphore key using the ftok() function, then gets a semaphore ID associated with that key using the semget() function (the first call also tests to make sure the semaphore exists). If the semaphore does not exist, the program creates it, as shown by the second call to semget(). In creating the semaphore for the queuing process, the program attempts to create one semaphore with read/write permission for all. It also uses the IPC_EXCL flag, which forces semget() to fail if the semaphore already exists.

After creating the semaphore, the program uses calls to semctl() and semop() to initialize it to the values in the sbuf array. The number of processes that can execute concurrently without queuing is initially set to 2. The final call to semget() creates a semaphore identifier that can be used later in the program.

Processes that obtain semid without creating it check that sem_otime is non-zero, to ensure that the creating process has completed the semop() initialization.

The final call to semop() acquires the semaphore and waits until it is free; the SEM_UNDO option releases the semaphore when the process exits, waiting until there are less than two processes running concurrently.

The POSIX Realtime Extension defines alternative interfaces for interprocess communication. Application developers who need to use IPC should design their applications so that modules using the IPC routines described in Section 2.7, XSI Interprocess Communication can be easily modified to use the alternative interfaces.

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, Copyright (C) 2013 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.unix.org/online.html .

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